1. Field of the invention
The present invention relates to spatial light modulators, more particularly, to full complex light modulators.
2. Description of the Related Art
Spatial light modulators (SLMs) are devices used to control the distribution of light in an optical system. Spatial light modulators are divided into one- or two-dimensional arrays of modulation elements called pixels, or picture elements, that represent the smallest addressable unit within the device. The SLM pixel is used to modify either the amplitude or the phase of the light distribution within the optical system.
In practice, the light modulation characteristics of most prior art SLMs are coupled combinations of amplitude and phase changes. The modulation characteristic of a pixel is controlled by a single applied signal, either an electrical voltage, current or incident optical intensity level, so the amplitude and phase characteristics of the pixel can not be independently set.
There are numerous applications, especially in optical information processing, in which controlling amplitude and phase independently is essential. Phase modulation is essential since most of the signal information is contained in the phase terms. The additional control of amplitude provides means for rejecting noise in the filter plane for improved system performance.
Four major types of modulators are presently being used for phase modulation; liquid crystal, photoretractive, magnetooptic, and deformable mirror. All have coupled phase and amplitude modulation characteristics.
Liquid crystals allow for phase and amplitude modulation, but phase modulation has extremely narrow ranges for the electric fields applied for uniform realignment, making it hard to control. Amplitude modulation is also difficult since the nonuniform realignment causing the amplitude modulation also contributes to phase modulation, resulting in a phase-amplitude coupled modulation.
Photorefractive modulators work for phase-only modulation only at extremely high voltages. Birefringence caused in nonuniform alignment produces amplitude modulation. But since photorefractive, like liquid crystal, modulates by a change in the refractive index, phase modulation accompanies amplitude modulation.
Magnetooptic modulators produce a binary change in the polarization of light, but are hard to control in operation. Kast, et al., in their article in Applied Optics, Mar. 15, 1989, describe a method for ternary operation of magnetooptic modulators, but it has a very limited range of amplitude- or phase-only modulations, none of which are independently controlled.
Present deformable mirror devices could be effective if the resolution of the optical system was fine enough to resolve the mirror element separate from the background. But, the normal setting for resolution of optical systems is the Nyquist frequency, causing the mirror to be mixed with the background. Amplitude modulation results from the interference between the two distributions.
Two other methods of phase-only modulation have been used. The first method was introduced by Brown and Lohmann in Applied Optics, 1966. Their technique, detour phase, requires very tight system alignment and limited field-of-view for the phase encoding approximations to be valid. The second was introduced by Hansche, et al., in their article in Applied Optics, Nov. 15, 1989. Their approach allows for different amplitudes to be produced, but requires a lowered resolution in the optical system.